188 DISCOURSE ON THE STUDY 



ation of great classes of phenomena, at length 

 come to be placed at issue upon a single fact. A 

 beautiful instance of this will be cited in the next 

 section. We may add to the examples above given 

 of such instances, that of the application of chemical 

 tests, which are almost universally crucial experi- 

 ments. 



(198.) Bacon's " travelling instances " are those 

 in which the nature or quality under investigation 

 " travels," or varies in degree ; and thus (accord- 

 ing to 152.) afford an indication of a cause by 

 a gradation of intensity in the effect. One of his 

 instances is very happy, being that of " paper, 

 which is white when dry, but proves less so when 

 wet, and comes nearer to the state of transparency 

 upon the exclusion of the air; and admission of 

 water." In reading this, and many other instances 

 in the Novum Organum, one would almost suppose 

 (had it been written) that its author^ had taken 

 them from Newton's Optics. 



(199.) The travelling instances, as well as what 

 Bacon terms " frontier instances," are cases in which 

 we are enabled to trace that general law which 

 seems to pervade all nature - the law, as it is 

 termed, of continuity, and which is expressed in the 

 well known sentence, " Natura non agit per sal- 

 turn." The pursuit of this law into cases where its 

 application is not at first sight obvious, has proved 

 a fertile source of physical discovery, and led us to 

 the knowledge of an analogy and intimate connec- 

 tion of phenomena between which at first we should 

 never have expected to find any. 



(200.) For example, the transparency of gold leaf, 



